Cisco MGX 8200 Series Edge Concentrators

October 22, 2003

Products Affected

Problem Description

Frame Relay Service Module (FRSM) sending Abit=0 when the Processor Switch Module (PXM) shows the channel to be clear. Traffic is interrupted until user intervenes by downing and upping the BPX connection segment. This condition is triggered by a softswitch.

Softswitch is used to activate the secondary card in order to release the primary card for maintenance or upgrade activity. This means that this anomaly applies only to FRSM cards that are part of a redundancy group.

Background

Root Cause : See also CSCdy38334

Needs any scenario where a lone Operation, Administration, and Maintenance (OAM) alarm indication signal (AIS) cell arrives from Network side. Once the OAM AIS cell is received, the alarm condition is latched and and it does not subside until the Local Management Interface (LMI) update is received.

Seed : Softswitch. All cards saw a softswitch during an upgrade.

Exists: Up to 1.2.11 including 1.1.32 -Base line Bug

Trigger : OAM AIS- not accompanied with A-Bit

Detail Explanation :

Step 1: Every time a softswitch is done on the FRSM card, the Permanent Virtual Circuit (PVC) status data structure gets initialized to an incorrect value. The PVC data structure will indicate that all connections on that FRSM card are receiving Abit=0 from the network.This structure is an internal structure, so the effects are not immediately visible. This condition stays dormant and unnoticed until a trigger causes this value to be copied over to the LMI status data structure.

Step 2: If now under this state, the remote end of a connection experiences a port failure then it will send out an AIS cell to the local end. The PVC status data structure on the local end now, because of the incorrect initialization as mentioned in step 1 will indicate that the card is receiving both the AIS and the Abit=0 from the network and this state is copied to the LMI data structure. LMI will now start to send out Abit=0 to the Customer Premise Equipment (CPE).

Step 3: The remote port failure clears and the far end stops sending AIS and the FRSM card stops receiving AIS from the network. The PVC data structure gets modified to indicate that AIS has stopped from the network. However the data structure still indicate that the Abit=0 is received from the network. Since there is still at least one apparent failure from the network abit=0, the LMI module continues to send Abit=0.

Problem Symptoms

The attached CPE continues receiving A-bit = 0 after connection alarms clear. Users may lose connectivity to remote resources and services.

Use the dspchancnt channel number command to observe A-bit state.

CHE1MGM1.1.13.FRSM.a > dspchancnt 200

ChanNum: 200
ChanState: alarm
ChanUpTime: 139389

Tx Rx
--------------- ---------------
AbitState: Sending A=0 Off
ATMState: Not sending any state Not receiving any state
Total Frames: 209930 221391
Total Bytes: 38502675 31680274
Frames DE: 87 0
Bytes DE: 102852 0
Frames Discarded: 0 0
Bytes Discarded: 0 0

Note the AbitState in the TX column above.

Workaround/Solution

Work Around for affected connections:

Once the connection is affected and not passing traffic, use Cisco WAN Manager (CWM) to reconfigure the committed information rate (CIR) of the connection. Or delete and re-add the affected PVCs.

Work Around upgrade:

Do not use softswitch when upgrading from 1.1.3x to 1.1.40, 1.1.42 or 1.1.42. Instead, delete 1:N redundancy and reset the inidividual service modules.

Solution:

If the upgrade work around presented above is not suitable, upgrade to 1.2.11 or a later 1.2 release instead of 1.1.4x releases.

DDTS

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For More Information

If you require further assistance, or if you have any further questions regarding this field notice, please contact the Cisco Systems Technical Assistance Center (TAC) by one of the following methods:

• Open a service request on Cisco.com
• By email
• By telephone

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